Automatic volume control circuits



.1. STEINMETZ 2,2@4,9?3

AUTOMATIC VOLUME CONTROL CIRCUITS Filed April 28, 1938 70 14. F. NETWORKINVEN TOR. JOACH/M STE/NMETZ A TTORIVEK Patented June 18, 1940 PATENTOFFICE AUTOMATIC VOLUME CONTROL CIRCUITS Joachim Steinmetz,Berlin-Tempelhof, Germany,

assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic mitbeschrankter Haftung, Berlin, Germany, a corporation of GermanyApplication April 28, 1938, Serial No. 204,723

In Germany 5 Claims.

This invention is concerned with a circuit organization designed toamplify the automatic volume control potential in receiver sets.

The object of such amplification is to raise the efiects of volumecontrol also in small receiver sets, say sets comprising only one radiofrequency stage, to such an extent that it will satisfy allrequirements. It is known in the prior art to I amplify the volumecontrol potential in a dis tinct tube, or else in a tube which alsoserves to amplify an alternating potential used in the op-= eration ofthe receiver apparatus. The practicing of such amplification of directcurrent potential is attended with serious difiiculties for the reasonthat fluctuations in working potential (supplied from battery or mains)occasion undesirable shifts in the control potential, with the resultthat the tube to be subjected to control action, for instance, may beshifted into the region of grid current flow. Moreover the amplifyingtube can not be replaced by a new tube without an incidental adjustmentof the worl ing voltages because of variations or fluctuations ofmanufacturing constants.

Now, the present invention is concerned with an improved type ofvolume-control (AVC) circuit in which the amplification of the controlpotential obtained by a rectification of the radio frequency energy iseffectedin a tube, with the amplified control voltage being taken off atthe cathode end of a resistance contained in the cathode circuit.

The invention more particularly resides in the feature that the diodeserving to provide the control potential (AVC potential) is connecteddirectly, or else through de-coupling resistances, between grid andfilament of the amplifier tube adapted to amplify the control potential,while the load resistance of the diode, which at the same time serves toimpress the grid biasing potential upon the amplifier tube, has its endaway from the grid, connected with a fixed potential of the receiverapparatus, more particularly a reference line rather than with thecathode of the amplifier tube.

quency energy (HF) is fed to the oscillation cir cuit S, and thencethrough the condenser C to the diode D which comprises the loadresistance B. The rectified potential serves to control the amplifiertube V, the amplified control potential being made available across thecathode resist- April 28, 1937 ance K thereof. In other words, thecathode of tube V experiences changes of its potential in relation to A,and thus, also, in reference to the reference line 0 to a more markeddegree than the anode of tube D in reference to point a. The amplifiedcontrol potential is then impressed upon the tube to be submitted tocontrol action through the AVG conductors.

Now, the operation of this circuit organization is as follows: First thecircuit elements S, C and D are assumed to be absent, that is, no radiofrequency energy is fed in. The remaining circuit elements constitute amarkedly stabilized direct current amplifier. A high degree ofstabilizing action is secured by the cathode resistance K, the latterbeing suitably chosen high, and, also, the potential across 0 and -Ashould be chosen correspondingly high. This stabilization is necessaryin order that by such fluctuations in the main supply voltage as mayhappen, no shift in the potential of the cathode of tube V, and thus ofthe steady biasing voltage of the tube to be regulated, may occur.

The stabilization operates in the following manner: If, for instance,the plate potential, and thus the plate current, of tube V rise as aresult of a fluctuation in the operating potential, the result is anincrease of potential at K. The potential prevailing at the lower end ofK in ref erence toa. acts through A, O, and B upon the grid, and sinceit represents a higher negative biasing potential it reduces again theplate current. If, on the contrary, the potential O in reference to Aexperienced an increase, then the negative biasing potential prevailingacross K would be diminished since both potentials are in seriesopposition. As a result the negative biasing potential of tube V isreduced. The plate current rises, and this occasions a higher voltage atK so that the negative biasing potential is caused to rise againapproximately to the original value. Since the biasing potential actingacross the bottom end of B and the cathode of tube V acts not only as abiasing voltage for tube V, but through the AVG leads acts upon the tubeto be controlled, also the biasing voltage of the latter is renderedunaffected by such fluctuations as may happen in the supplyline voltage, and these would become extremely annoying in the absence of thecathode resistance K.

Assuming parts S, C, and D to be existing, it will first be noticed thaton the anode of the diode D there prevails the same negative biasingpotential acting as a retarding, or delay, voltage for thevolume=control as the voltage im- 553 pressed upon the grid of tube Vand acting between the AVG conductors. This biasing voltage is equal tothe difference between the potential across the resistance K and thevoltage applied across 0 and A, and is obtained by choosing suitablevalues for the resistance K.

If an impressed radio frequency potential sunpasses the delay potential,rectification is had and the anode of D becomes more negative. If, then,the cathode of the diode D were connected with the lower end of the loadresistance B rather than with the cathode point a of tube V, as isusually the case, the direct voltage furnished from the rectifier andits effect upon the amplifier tube V would be compensated again, in thevery same manner as if the voltage of point 0 relative to A had beenaltered. However, according to the invention the cathode of the diode Dand the bottom end of resistance B, as shown in Fig. 1, are connected todifferent points. The practi cal result is that the stabilization of theamplifier stage is left unaffected, while yet amplification of thecontrol voltage is brought about. In other words, a counter, or inverse,regulator or control action is produced for fluctuations of theoperating voltage, whereas for the volume-control potential which is tobe amplified, there occurs no inverse, or counter, control action, butmerely the normal amplification. The underlying,- reason is this: Whenthe rectified, negative voltage impressed upon the grid of tube Vdecreases the plate current of tube V, the voltage across K drops, and,as a consequence, there occurs a. decrease of the potential acting onthe cathode of tube V and thus, also, on the cathode of diode D into thenegative region.

As a result, the voltage across the cathode of tube D and the bottomterminal of resistance B having a negative action for the anode of thediode is reduced, while, as a consequence, the diode current is raised.The increase in the diode current, however, results in a higherpotential across the resistance B, and this, in turn, causes the anodeof the diode to be subjected to a more negative biasing potential. Thecounter-action due to the decrease of the potential at the cathode oftube V, as will thus be seen, is again compensated and cancelled outalmost completely by the resulting stronger diode current. One couldalso conceive of this process in a way as if the potential of the gridof V is forced to fall in step (is entrained or driven) by the diodecurrent of D upon the occurrence of variations of potential of a.

Fig. 2 illustrates an exemplified embodiment of this circuit in whichthe tube V, in a reflex circuit scheme, for amplification of both radioand audio energies, is further relied upon to amplify the volume-controlvoltage. The incoming radio frequency energy HF is subjected toaperiodic amplification in tube V and is rectified in the righthanddiode to result in audio voltage being fed back to the tube V and beingtaken oif at the anode in amplified form, and. then fed forward to theaudio power stage. In the left-hand diode D is produced the controlpotential which is impressed through two filter resistances W upon thegrid of the tube V. The amplified control potential arises across thecathode resistance K, and is thence impressed upon the RF tube to becontrolled. The choke-coil D1 serves to correct the irregularities dueto decrease in the gain at the higher frequencies (about 1000 kc.) inthat it forms a condition of resonance together with the diode capacity.The coupling capacity 2 of 50 mmf. (micromicrofarads) together with thechoke 3 of 100 mh. precludes audio energy from the diode.

In measurements made with this circuit, the voltage of the cathode oftube V in reference to the reference lead 0 which is equal to the gridbiasing voltage of tube V, was found to experience a shift of only i035v., in the presence of a variation of all operating potentials.

Taking into consideration all possible tube tolerances, aging of tubesand a :10% supply-line voltage variation, the maximum departures fromthe normal or rated grid biasing voltage was found to be 0.5 to +0.8 v.by actual measurement.

Insofar as the control action is concerned, the following situation wasfound: In the presence of a 1:3 transformation of the antenna potentialat the oscillation circuit of the only tube to be controlled, and a gainin this tube of 330 maximum, the control curves evidenced a nearly idealshape, for the point of incipient control and simultaneously thefull-load operation of the power stage were around 100 microvolt inputpotential. From this point up to 100 millivolt (equal to a ratio of1:1000) the output voltage was found to vary only as 1:2. When thecontrolled tube was of the hexode type, practically no overload limitwas found to exist, whereas with a pentode the overload limit lay around150 millivolts.

The bottom end of resistor K is suitably impressed with a voltage thatis markedly negative in reference to the reference line of the receiver,say, -100 v. Such a high voltage, for instance, will be present when thefield coil of the loudspeaker is used as a choke. It will be noted thatthe following constants for the circuit of Fig. 2 are given merely forillustration:

W: 1 M-ohms W: 0.5 M-ohms Resistor 4: K-ohms Resistor 5: 200 K-ohmsResistor 6= 2 M-ohms Resistor '7: 100 K-ohms B: 200 K-ohms K=1600-170Oohms Condenser 8: 6 mi.

D1: 1 mh.

What is claimed is:

1. In combination with a signal amplifier of the type having a tubeprovided with a least a cathode, grid and anode, a signal input circuitconnected between the grid and cathode, a resistive impedance in thespace current path of said tube and connected between the cathode and apoint of negative potential, a second resistive impedance connectedbetween the grid and a potential point which is positive with respect tosaid negative point, said first impedance having a sufficiently largevalue to cause the cathode to be positive with respect to said grid, anda rectifier connected in circuit with said second impedance whereby thedirect current voltage developed across the latter by rectified signalsis impressed on said grid for direct current amplification, a secondrectifier, means for impressing amplified signals in the amplifier anodecircuit upon the second rectifier, means impressing the audio voltageoutput of the second rectifier upon the amplifier tube grid whereby saidamplifier acts as an audio amplifier.

2. In combination with a signal amplifier tube having at least acathode, a signal grid and. an

output plate, means responsive to space current amplitude of the tubefor establishing the oathode at a positive potential with respect to afixed potential point, a resistor connecting the grid to said fixedpoint to establish the grid at a negative bias, a signal rectifiercircuit comprising a diode including said resistor in circuit therewithas the load impedance, direct current voltage variations developedacross said resistor due to signal rectification being amplified by saidtube, and means for deriving amplified direct current voltage variationsfrom said responsive means.

3. In combination with a signal amplifier tube having at least acathode, a signal grid and an output plate, means responsive to spacecurrent amplitude of the tube for establishing the oathode at a positivepotential with respect to a fixed potential point, a resistor connectingthe grid to said fixed point to establish the grid at a negaative bias,a signal rectifier circuit comprising a diode including said resistor incircuit therewith as the load impedance, direct current voltagevariations developed across said resistor due to signal rectificationbeing amplified by said tube, and means for deriving amplified directcurrent voltage variations from said responsive means,

said diode having its electrodes connected to said responsive means sothat the diode is normally non-conductive.

4. In combination with a tube, means to impress radio energy thereon foramplification, rectifier means to derive an audio voltage from theamplified energy, means to impress the audio voltage on the tube foraudio voltage amplification, a second, separate rectifier means derivinga direct current voltage from the amplified radio energy, meansimpressing the direct current voltage upon said tube for direct currentvoltage amplification, and means for utilizing the amplified directcurrent voltage.

5. In combination with a tube, means to impress radio energy thereon foramplification, rectifier means to derive an audio voltage from theamplified energy, means to impress the audio voltage on the tube foraudio'voltage amplification, a second, separate rectifier means derivinga direct current voltage from'the amplified radio energy, meansimpressing the direct current voltage upon said tube for direct currentvoltage amplification, means for utilizing the amplified directcurrentvoltage; and means for utilizing the amplified audio voltage.

JOACHIM STEINMETZ.

